Though carbon nanotubes are the best known examples of one-dimensional (1-D) systems, the synthesis and characterization of metal oxide nanotubes and nanowires are equally critical because of the intrinsic importance of 1-D structures as model systems for the efficient transport of electrons and optical excitations (Hu et al., Acc. Chem. Res. 1999, 32, 435-445; Ijima, S. Nature 1991, 354-56-58). The quantum confinement and low-dimensionality inherent to these systems allow for the generation of materials with unique properties, such as a higher luminescence efficiency (Holmes et al., Science 2000, 287, 1471-1473) and a lowered lasing threshold (Arakawa et al., Appl. Phys. Lett. 1982, 40, 939-941) as compared with the bulk. Moreover, 1-D systems can be used as building blocks for the next generation of nanoscale optical, electronic, photoelectrochemical, and photovoltaic devices (Cui et al., Science 2001, 291, 851-853; Huang et al., Science 2001, 292, 1897-1899). A lot of effort has been expended in overcoming numerous challenges associated with the efficient design of 1-D materials possessing well-defined and reproducible size, shape, monodispersity, purity, chemical composition, and crystallinity (Xia et al., Adv. Mater. 2003, 15, 353-389). The binary oxides of transition metal elements, such as Zn, Cu, and Fe, in particular, represent one of the most diverse classes of materials with important size-dependent optical, electronic, thermal, mechanical, chemical, and physical properties, with a wide range of correspondingly diverse applications, including energy storage and sensing (Fan et al., Appl. Phys. Lett. 2004, 85, 5923-5925; Johnson et al., Nano Lett. 2004, 4, 197-204; Law et al., J. Phys. Chem. B 2006, 110, 22652-22663; Johnson et al., J. Phys. Chem. B. 2001, 105, 11387-11390; Law et al., Nat. Mater. 2005, 4, 455-459; Sun et al., Adv. Mater. 2005, 17, 2993-2997; Chen et al., Adv. Mater. 2005, 17, 582-586; and Zhu et al., Nanotechnology 2005, 16, 88-92).
ZnO is a key, II-VI compound semiconductor, with particularly attractive properties such as a direct wide band gap (3.37 eV), a large exciton binding energy (60 meV at room temperature), and an exciton Bohr radius in the range of 1.4-3.5 nm (Reynolds et al., Phys. Rev. B 1998, 57, 12151-12155). Moreover, ZnO possesses a high breakdown voltage, good piezoelectric characteristics, biocompatibility, as well as high mechanical, thermal, and chemical stability. All of these favorable properties render this material highly versatile for a host of optoelectronic applications including room-temperature ultraviolet lasers (Huang et al., Science 2001, 292, 1897-1899), photodetectors (Kind et al., Adv. Mater. 2002, 14, 158-160; Soci et al., Nano Lett. 2007, 7, 1003-1009), dye-sensitized solar cells (Law et al., Nat. Mater. 2005, 4, 455-459; Suh et al., Chem. Phys. Lett. 2007, 442, 348-353), and field-effect transistors (Goldberger, et al., J. Phys. Chem. B 2005, 109, 9-14; Noh et al., Appl. Phys. Lett. 2007, 91, 043109-1-043109-3; Wang et al., Adv. Mater. 2007, 19, 1627-1631). ZnO nanowire arrays have been utilized as field emission sources as well as power generators for nanoscale devices (Wang et al., Science 2006, 312, 242-246; Gao et al., Adv. Mater. 2007, 19, 67-72; Wang et al., Nano Lett. 2007, 7, 2475-2479).
ZnO nanowires and nanowire arrays have been previously synthesized via both vapor and solution phases. Typical approaches were based on metal-organic chemical vapor deposition (MOCVO) (Park et al., Adv. Mater. 2002, 14, 1841-1943; Jeong et al., Nanotechnology 2006, 17, 526-530), chemical vapor transport (CVT) (Yang et al., Adv. Funct. Mater. 2002, 12, 323-331; Umar et al., Nanotechnology 2007, 18, 175606-1, 75606-7), and pulsed laser deposition (PLD) (Sun et al., Chem. Phys. Lett. 2004, 396, 21-26). These methods, while fully capable of generating high-quality wires and arrays, do possess limitations. For instance, gas-phase methods tend to involve the use of high temperatures (e.g., 450-900° C.), potentially toxic precursors, and a very limited range of substrates in order to induce and direct the growth of ZnO nanowires. Moreover, PLD is not an inexpensive method of producing ZnO nanostructures. Solution-based methodologies also exist for ZnO formation (Vayssieres L., Adv. Mater. 2003, 15, 464-466). As an illustrative example, layers of ZnO seed nanocrystals, measuring 5-10 nm in diameter, can be initially formed onto a Si substrate by thermally decomposing zinc acetate at 200-350° C., and this 50-200 nm film of crystal seeds can be subsequently grown into vertical nanowire arrays at 90° C. (Greene, et al., Chem., Int. Ed. 2003, 42, 3031-3034; Greene et al., Nano Lett. 2005, 5, 1231-1236). Recently, this methodology was extended to the growth of aligned ZnO nanowire arrays on a plastic film using Au nanocrystal seeds (Gao, et al., Adv. Mater. 2007, 19, 67-72).
As a p-type semiconductor with a narrow band gap (1.2 eV), CuO is a candidate material for photothermal and photoconductive applications (Jiang et al., Nano Lett. 2002, 2, 1333-1338; Musa et al., Sol. Energy Mater. Sol. Cells 1998, 51, 305-316). Moreover, it is also an effective heterogeneous catalyst (Reitz et al., J. Am. Chem. Soc. 1998, 120, 11467-11478) for converting hydrocarbons completely into carbon dioxide and water. In addition, it is potentially a useful component in the fabrication of sensors, magnetic storage media, field emitters, lithium-copper oxide electrochemical cells, cathode materials, and high Tc-superconductors (Lanza et al., J. Mater. Res. 1990, 5, 1739-1744; Podhajecky et al., Electrochim. Acta 1990, 35, 245-249). CuO nanowires can, for instance, be synthesized merely by heating Cu substrates in air from 400 to 700° C. (Jiang et al., Nano Lett. 2002, 2, 1333-1338; Cheng et al., Nanotechnology 2007, 18, 245604-1-245604-5), while 1D CuO nanostructures can be obtained by a high-temperature transformation of their 1D copper hydroxide nanoscale analogues (Cao et al., Chem. Commun. 2003, 1884-1885; Wen et al., Langmuir 2005, 21, 4729-4737; Du et al., Chem. Phys. Lett. 2004, 393, 64-69; Lu et al., J. Phys. Chem B 2004, 108, 17825-17831; Zhang et al., J. Cryst Growth 2006, 291, 479-484). Polycrystalline CuO nanofibers have been prepared through electrospinning (Wu et al., Appl. Phys. Lett. 2006, 89, 133125-1-133125-3). Free standing CuO nanotube and nanowire arrays have been fabricated by depositing precursors of either a MOCVD process (Malandrino et al., Chem. Mater. 2004, 16, 5559-5561) or a sol-gel technique (Yi-Kun et al., Trans. Nonferrous Met. SOC. China 2007, 17, 783-786) into the uniform pores of alumina templates, followed by subsequent annealing.
Because of its high stability, relatively low cost, and n-type semiconducting properties with a small bandgap (2.1 eV), α-Fe2O3 has been associated with applications ranging from gas sensing, lithium-ion battery production, catalysis, water splitting, water purification, and solar energy conversion to pigmentation (Chen et al., Adv. Mater, 2005, 17, 582-586; Gondal et al., Chem. Phys. Lett. 2004, 385, 111-115; Ohmori et al., Phys. Chem. Phys. 2000, 2, 3519-3522). Nanobelts, nanowires, and arrays of hematite structures have been synthesized by different methods, such as (a) the direct thermal oxidation of a pure iron substrate in an oxidizing atmosphere with a temperature range of 500-800° C. (Fu et al., Chem. Chem. Phys. Lett. 2003, 379, 373-379; Wen et al., J. Phys. Chem. B 2005, 109, 215-220); (b) the vacuum pyrolysis of β-FeOOH nanowires in a pressure range of 10−2 to 10−3 atm (Wang et al., J. Mater. Chem. 2004, 14, 905-907; Xiong et al., Inorg. Chem. 2004, 43, 6540-6542); and (c) PLD using pressed Fe3O4 powder as a target (Morber et al., J. Phys. Chem. B 2006, 110, 21672-21679). Hematite nanotubes (Chen et al., Adv. Mater. 2005, 17, 582-586) and their corresponding arrays (Shen et al., Chem. Lett. 2004, 33, 1128-1129) have also been obtained by decomposing organometallic iron precursors, embedded within the pores of an alumina template, at high temperature. Moreover, hematite nanotubes have been synthesized by using carbon nanotubes as a structural template motif (Sun et al., Adv. Mater. 2005, 17, 2993-2997).
It would be desirable to develop a protocol that allows for an environmentally sound and cost-effective methodology of metal oxide nanoscale synthesis without the need to sacrifice on sample quality, crystallinity, monodispersity, and purity. For example, it would be an advance to develop a generalizable protocol aimed at ZnO, CuO, and α-Fe2O3 (hematite) nanowire/array formation while overcoming the high temperatures, the need for expensive equipment, the use of potentially toxic precursors and byproducts, or the ultimate product polycrystallinity, characteristic of previous methods of metal oxide nanoscale synthesis.